Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 94

FIGURE 6.3 The van 't Hoff plots of several selected metal hydrides. Source : Reproduced with permis-
sion from Zuttel [3]. (See color insert.)

[3]. The dehydrogenation process is a reverse process as shown in Figure
6.2, and it also shows a desorption plateau with a lower near-constant hydro-
gen pressure P D . The adsorption and desorption plateaus form a hysteresis
loop for the hydrogenation and dehydrogenation processes, and the free
energy difference associated with the hysteresis is given by

P
P

A

∆ G

hyst =

RT ln . (6.6)

D

The hysteresis represents a loss in the efficiency of the hydride due to
irreversible degradations of materials during hydrogenation/dehydrogenation
processes. A good hydride material should have a hysteresis as small as
possible.
The width of the plateau, Δ( H / M ) r , as indicated in Figure 6.2, is defined
as the reversible capacity, which is usually smaller than the maximum (ideal)
storage capacity discussed earlier. This is an important parameter for practi-
cal applications.

6.2.3 Hydrogenation and Dehydrogenation Kinetics

The rates of hydrogen adsorption and desorption are two very important
parameters for characterizing metal hydrides. These rates depend on the